1. Field of the Invention
The present invention relates to a thermal head which provides uniform density in an image portion near an end of an array of heating resistor elements.
2. Description of the Prior Art
A conventional thermal head is shown in FIG. 1. In the figure, reference characters D.sub.1, D.sub.2, . . . D.sub.7 respectively denote heating resistor elements; C, a common feeder pattern to the respective heating elements; L.sub.1, L.sub.2, . . . L.sub.7, individual feeder patterns to the respective heating elements. Reference numeral 1 denotes a whole thermal head which includes the respective heating resistor elements and the feeder patterns. In such thermal head, a source power is applied across the common feeder pattern C and each of the individual feeder patterns L.sub.1, L.sub.2, . . . L.sub.7 to cause the resistor elements D.sub.1, D.sub.2, . . . D.sub.7 to generate heat, thereby coloring heat-sensitive paper (not shown) for thermal printing.
Both end elements D.sub.1, D.sub.7 of these resistor elements D.sub.1 to D.sub.7 produce light printing density and have short service life compared to the inner elements D.sub.2 to D.sub.6. This is mainly because the inner heating resistor elements D.sub.2 to D.sub.6 each have a heating resistor element at each end thereof which generates heat, thereby reducing thermal diffusion from the inner elements D.sub.2 to D.sub.6 toward the respective adjacent end elements whereas the end elements D.sub.1, D.sub.7 each have heating resistors only on one side, so that heat will diffuse in the direction in which there are no heating resistor elements. For example, when the second heating resistor element D.sub.2 generates heat, this heat is difficult to diffuse toward the resistor elements D.sub.1, D.sub.3 and to diffuse toward the feeder patterns C, L.sub.2 on either side of the element D.sub.2 (in the respective directions A and B). In comparison with this, the upper end heating resistor element D.sub.1, generates heat which further diffuses in the X direction. Similarly, heat from the lower end heating resistor element D.sub.7 diffuses in the Y direction. If thermal diffusion is severe as described above, the upper and lower end elements provide a clearly degraded printing density, and have rapid temperature rise and fall curves, which can shorten their service life. As a potential solution to these problems, the inventor conceives that unillustrated auxiliary heating resistor elements (the same structure as, or quite different in resistance from, the main heating resistor elements) could be provided separately at the upper and lower ends of the main heating resistor elements D.sub.1, D.sub.2, . . . D.sub.7 to prevent upward and downward heat diffusion from the main upper and lower end heating resistor elements, when the power is applied to the upper and lower ends of the main heating resistor elements. Furthermore, energy in an amount which hardly creates color could be applied to the auxiliary heating resistor elements, thereby providing uniform printing density and prolonged service life.
According to this method, however, additional output lines l.sub.0, l.sub.8 are required for driving the above auxiliary heating resistor elements in the schematic connection diagram of FIG. 2. This leads to enlarged dimension and raised cost of the thermal head due to an increase in the number of feeder patterns, raised cost due to an increase in the number of pins in the connector CN1 for connecting the drive circuit DC1 with the thermal head 1, and complication of the electric circuit and drive method concerned due to the necessity of using two kinds of heat pulse.